1. Field of the Invention
The present invention relates to an image signal processing device, and more particularly, to an automatic fine tuning (hereinafter, referred to as AFT) circuit used for processing an image intermediate frequency signal for a video device.
The instant application is based on Korean Patent Application No. 16852/1994, which is incorporated herein by reference for all purposes.
2. Discussion of Related Art
An AFT circuit produces an AFT voltage, for performing the AFT operation of detecting a variation of a carrier frequency of a received image intermediate frequency signal. The AFT voltage is passed through a lowpass filter and then is fed back to a local oscillator of a tuner. The local oscillator of the tuner varies an oscillation frequency so that the image intermediate frequency signal can be kept as an exact carrier center frequency fp (45.74 MHz in a NTSC system) by the AFT voltage.
FIG. 1 is a block diagram illustrating a prior art AFT circuit. An image intermediate frequency signal Vi is received by a phase shifter 10 and a phase detector 20, respectively. The phase shift of the phase shifter 10 depends on the inputted frequency. A phase shift of 90.degree. is performed only when the carrier frequency of the image intermediate frequency signal Vi corresponds to the carrier center frequency fp. An image intermediate frequency signal Vo, which has been phase-shifted in the phase shifter 10, is subsequently applied to the phase detector 20.
FIG. 2 is a graph illustrating the characteristic of the phase shift of phase shifter 10 corresponding to the carrier frequency of the image intermediate frequency signal Vi, received by phase shifter 10.
Phase detector 20 multiplies the image intermediate frequency signal Vi by the phase-shifted image intermediate frequency signal Vo. As a result, a voltage indicative of a phase difference between the two signals (the AFT voltage) is produced. The AFT voltage is then passed through a lowpass filter, and is subsequently fed back to the local oscillator of a tuner.
FIG. 3 is a block diagram illustrating a detailed circuit of the phase shifter 10 of FIG. 1. The received image intermediate frequency signal Vi is amplified by differential amplifiers Q1 and Q2, and then is passed through capacitors C1 and C2, thereby being received by an AFT resonance circuit 15.
The AFT resonance circuit 15 has a parallel structure, comprising a variable coil L1 connected between the capacitors C1 and C2, a capacitor C3 connected in parallel to the variable coil L1, resistors R1 and R2 respectively connected to both terminals of the capacitor C3, and a bias voltage supplying power source V1 connected to the resistors R1 and R2. Here, the AFT resonance circuit 15 is adjusted by the variable coil L1, so that when the carrier frequency of the image intermediate frequency signal Vi exactly equals a resonant frequency fp, a phase shift of 90.degree. can be performed. Under such a circuit configuration, the phase shifter 10 has the same characteristic as that shown in FIG. 2.
In the AFT circuit as shown in FIG. 1, the carrier frequency of the received image intermediate frequency signal Vi is very high. For this reason, it is preferable that a tuning circuit used in the AFT circuit should have excellent selectivity and stability.
A phase shifter 10 using the AFT resonance circuit 15, comprising the variable coil L1, the capacitor C3, and the resistors R1 and R2, allows for exact detection of the carrier center frequency of the image intermediate frequency signal Vi. However, even though the prior art AFT circuit operates well alone, the recent trend pursuing system integration has created problems that are described below.
One problem associated with system integration is that often the distance between the a voltage controlled oscillator (VCO) resonance circuit (which detects a phase locked loop (PLL)image for processing an image intermediate frequency signal) and the AFT resonance circuit is very short. As a result, a mutual interference can be generated between the VCO coil of the VCO resonance circuit and the AFT coil of the AFT resonance circuit. Furthermore, a signal level in the AFT coil of the AFT resonance circuit can induce a corresponding signal in the VCO coil for detecting the image, thereby generating a mis-oscillation of the VCO. Due to the mutual interference as described above, a quadrature phase shifter 10, as shown in FIG. 1, cannot phase shift the image intermediate frequency signal Vi by 90.degree. when it corresponds to the carrier center frequency fp.